dpdk: Add iova-mode to startup

In some cases of using vfio-pci driver, it should be required
to enable pa or va mode. Without it, rtl_eal_init unable to
allocate memory required. Debugging told that iova-mode pa/va
could help. And it helps.

This patch allows to pass iova-mode to vpp startup.conf

Type: feature
Change-Id: I36b87f5d3d141891b37cda2c306d50433954a34a
Signed-off-by: Vladimir Ratnikov <vratnikov@netgate.com>

src/plugins/dpdk/device/dpdk_priv.h

@@ -47,7 +47,8 @@ _(huge-dir)                                     \
 _(proc-type)                                    \
 _(file-prefix)                                  \
 _(vdev)                                         \
-_(log-level)
+_(log-level)                                    \
+_(iova-mode)
 
 static inline void
 dpdk_get_xstats (dpdk_device_t * xd)